GOHMISSION GEOLOGIQVE - Arkisto.gsf.fi

Suomen Geologinen Seura. N:o 12. Ceologiska Sallskapet i Finland. 23
writes: ))The tortuous side-veins shown in sketch A likewise traverse
micaceous bands in the granuliteu p. 76. But although the injection
here followed along the direction of the bedding it is ptygmatic. Read
points out that ptygmatic veins are more common in the metamorphic
8hardenedu rocks of the injection-complex proper than outside it.
But to attain the present hardened structure the rocks had first to be
heated until recrystallization or annealing took place. The ptygmatic
veins belong to the close of the metamorphosing process and were
therefore injected not into a hardened, but into a softened surrounding
as Sederholm pointed out. Point 8 against Read is easily explained
by the folding theory. The anticlinal limbs are at right
angles to the compression and have been attenuated, the contortions
in the crest are in the direction of the pressure.
Where the ptygmatic folds follow a general direction parallel to
the schistosity (fig. 2, A) we must assume that the orientation of the
rock-complex has changed with respect to the compression so as to
reverse the sense of shortening. This may happen in strongly kneaded
rocks, with overturned folds, etc. When it is parallel to the original
bedding the change of orientation need not have occurred.
The combination of ptygmatic folds and phenomena denoting
stretching are not uncommon. Fig. 2, C is an example. Following
Sander, the lengthening at right angles to the compression can be read
off from the broken and stretched basic stratum and pegmatitic veins.
The arrows show the movements that may elucidate this case, but
they were not necessarily in the plane of the section.
During compression the entire mass was at a high temperature.
The country rock was slightly more plastic, due either to smaller grain
or to Chemical and mineralogical composition. The plasticity may
have been due to recrystallization, especially when we are dealing with
micaceous schists.
Sander supposed that the country rock was not exactly of the same
degree of plasticity throughout. One might liken it to illmixed dough.
The consequence of strong movements would be that some portions
are deformed, while shortly after a neighbouring part was more severely
distorted. In this manner cornplicated cases could arise of ptygmatically
folded veins, cut by a straight younger generation. Subsequent
annealing would obliterate evidence of the sequence.
In some cases, such as that described by Sederholm from the rapakivi
contact in the Pellinge region, the deformation must be due to
pressure exerted thmugh the intruding magma.
It is sometimes found, that thick ptygmatic veins, for instance
fig. 1, E, are highly irregular. This may be because the country rock